Watch her eyes. Does she make eye contact before and after the kiss? This indicates she feels a connection. Were her eyes open while you were kissing? This is a sign that she wasn't feeling the passion.
Watch where she places her hands. Hands with closed fists at her sides tell you she wants the kiss to end. Open hands placed on your lower back, hip or arm tells you she wants to get closer.
Notice how she positions her body while you kiss. Is she leaning closer to you with her whole body? She is telling you that she is enjoying the kiss and feeling a connection. Hugging you with her entire body while kissing is a good sign. If she is pulling her body back and distancing herself from you, she is not into you.
Kissing you quickly with a closed mouth indicates she is not into the kiss. If she presses her lips more firmly against your lips during the kiss, she is feeling the fire. Allowing her lips to linger close to your lips after the kiss indicates she wants to kiss again.